New Type of Black Hole May Offer Galactic Insight

By JOHN NOBLE WILFORD

Published: September 18, 2002

The Hubble Space Telescope has detected the first clear evidence for a new category of cosmic black holes, astronomers reported yesterday. The discovery is expected to yield insights about the evolution of black holes and the formation of star clusters and galaxies in the early universe.

In a briefing at the National Aeronautics and Space Administration in Washington, the astronomers described finding the first two medium-weight black holes, the new class of these extremely dense but titanic gravitational sinks. One has a mass 4,000 times that of the Sun; the other, 20,000 times.

It was not a complete surprise. The Chandra X-ray Observatory, which orbits Earth, had delivered tantalizing clues that there could be intermediate black holes: something much heavier than those created by the collapse of a single huge star but nothing like the supermassive ones that weigh as much as billions of stars and roil the cores of most galaxies.

But astronomers had to look in unexpected places — at the centers of bright star groups known as globular clusters — to find the new class of black holes. These clusters contain the oldest stars in the universe and are relatively benign environments, which suggested to scientists that the black holes were formed at about the same time as the clusters themselves.

"These findings may be telling us something very deep about the formation of star clusters and black holes in the early universe," said Dr. Roeland van der Marel of the Space Telescope Science Institute in Baltimore.

Dr. Steinn Sigurdsson, an astronomer at Pennsylvania State University, said medium-weight black holes might constitute a "missing link" in the mysteries of galactic origins and evolution.

Dr. Michael Rich, an astronomer at the University of California at Los Angeles, said, "Not only will we learn about the formation of the black holes, but these new data from Hubble help us connect globular clusters to galaxies, providing information on one of the most important unsolved problems in astronomy today: how galaxy structure forms in the universe."

The lighter of the two black holes was found by a team led by Dr. van der Marel. The hole is at the center of the globular star cluster M15, which is 32,000 light-years away in the constellation Pegasus, at the edge of Earth's home galaxy, the Milky Way.

The heavier one, detected by a group led by Dr. Rich, is farther away, in the giant globular cluster G1. That is 2.2 million light-years from Earth, in the neighboring Andromeda galaxy.

By their very nature — a gravitational power so great that nothing, not even light, can escape their clutches — the black holes could not be observed directly. But the Hubble Telescope was able to focus on individual stars in the clusters and track their velocities as they orbited close to the clusters' cores. Increased velocities indicated that they were being attracted and accelerated by the gravity of black holes.

Finding mediumweight black holes in some of their first searches indicated to astronomers that similar objects, and black holes in general, are probably more common in the universe than had been thought. It was not clear yet, astronomers said, whether all or even most globular star clusters harbor these intermediate black holes.

One of the most intriguing aspects of the two newly discovered black holes, astronomers said, is that their masses are proportional to the masses of the clusters they inhabit. Coincidence, or important clue?

It so happens that supermassive black holes already studied in the heart of galaxies represent about five-tenths of 1 percent of the masses of their host galaxies. That turns out to be the same mass proportion that mediumweight black holes have to their host star clusters.

"Whenever you see a relationship in astronomy, there is almost always some underlying cause," said Dr. Karl Gebhardt, an astronomer at the University of Texas. "We really don't know what is going on here yet, but it is telling us something about how globular clusters formed and how galaxies formed."